Several systems have been developed for, or adapted to, HDD that successfully directionally drill through mixed soil conditions and rock. This article looks into slant face rock bits and rotary steerable HDD systems. Several case studies are examined, and numerous references are cited, as well as factors to consider in selecting rotary rock bits for HDD applications, along with a bit selection chart. The importance of site investigaion at the design and pre-bid stage of a project is stressed. As a result, the project owner, engineer, and contractor will have better knowledge of HDD equipment options suitable for product installation in difficult ground conditions.

Please view the attached white paper at: http://www.ospmag.com/files/pdf/HDDSystemsforMixedConditions-111508.pdf